Coatings of dibutyl tin difluoride (DBTF) powder on glass are frequently employed with a view to reinforcing their surface, staining it and providing such surfaces with specific optical and/or electrical characteristics, most notably, low emissivity. The use of powdered DBTF to obtain such a coating with low emissivity, is described in European Pat. No. 39 256.
DBTF is conventionally manufactured from dibutyl tin dichloride (DBTCl) and potassium fluoride in a wateralcohol medium, according to the reaction: EQU (Bu).sub.2 SnCl.sub.2 +2 KF.fwdarw.(Bu).sub.2 SnF.sub.2 +2KCl
However, the DBTF thus obtained contains potassium as an impurity. It is also known that DBTF can be synthesized from sodium fluoride or other alkaline or alkaline-earth fluorides, however the DBTF obtained by this process contains sodium and, in general, an alkali or an alkaline earth as impurities.
The alkaline earth or the alkali, such as sodium or potassium, when encountered in the pyrolyzed coating, captures any available free electric charges. This results in a reduction of the optical and electrical characteristics of the coating in relation to those of a coating which is free of these impurities.
Furthermore, the DBTF powder is usually applied to a substrate to be coated, such as, e.g. a glass strip coming out of a float bath, by means of a nozzle of the type described in French patent applications Nos. FR 83/4,124 and 83/4,125. The coating will be of a uniform quality if the powder flows well in the distributing and proportioning devices above the nozzle and in the nozzle itself, but prior art DBTF powders often tend to become agglomerated at these points. The alkaline-earth or alkaline impurities, particularly, sodium or potassium, which are present in the powder, render it hygroscopic, and this impairs its flowability due to the consequent agglomeration which occurs. The agglomeration, in turn, is detrimental to obtaining a homogeneous coating due to its obstruction of powder flow.
In order to remedy this flow problem, it is possible to eliminate these impurities by one or usually more additional operations, such as a purification step, but this is a lengthy and expensive procedure.